1. Field of the Invention
This invention relates to semiconductor lasers, and more particularly to devices for polarizing and compensating the laser beam for nonuniform intensity distributions.
2. Description of the Prior Art
In recent years, semiconductor laser devices have become of increasingly greater commercial interest. For many applications of semiconductor lasers, it is necessary that the light beam generated by the laser have a substantially circular cross-section. One example of such an application is the optical disk wherein information is recorded on a disk at high density. To record or play the information back, a light beam from the semiconductor laser device must be formed into a light spot of about 1 micron in diameter on the disk. In general, however, the semiconductor laser has a rectangular light emitting region. The collimated beam will, however, have a nonuniform intensity distribution, having an oval or elliptical shape. The characteristics of the ellipse will depend upon the particular semiconductor laser. In order to efficiently store or accurately retrieve information from an optical disk, it is desirable to correct the oval shaped intensity distribution of the beam to a circular distribution.
It is also desirable for the laser beam to be polarized to minimize "backtalk" or interference caused by reflected radiation.
U.S. Pat. No. 4,084,883 discloses a mirror system on a laser material for polarizing laser radiation. Laser material, for example, neodymium doped glass is coated with twenty-five thin film layers of alternately a high-index and a low-index refraction material of a quarter-wavelength optical thickness. The low-index material is suggested as silicon dioxide and the high-index material is suggested to be within the range of 2.0-2.2 such as titanium dioxide (column 3, lines 48-60 and column 5, lines 6-45). A phase adjustor of one or several layers is then mounted on top of the multilayer coating and finally a reflector, which again can consist of multiple layers of thin coating, is deposited on the phase adjustor.
U.S. Pat. No. 4,173,036 discloses the use of a "wavy" mirror to reflect a beam from a laser diode in a thin conical fan distribution of uniform intensity.
U.S. Pat. No. 4,312,570 patent discloses the use of high- and low-optical quarter-wave coating on a reflective surface to provide polarization of reflected light.
U.S. Pat. No. 2,669,902 discloses the use of cemented 45.degree. prisms with quarter-wavelength coatings to provide a polarizing beam splitter.
One known quarter-wavelength multiple layer polarizer utilizes a low refractive index material (N.sub.L =1.38) and a high refractive index material of (N.sub.H =2.3), typically Z.sub.n S. As the number of layers increases, however, the degree of reflection of the TM mode of the incident laser radiation increases, reducing the efficiency of the polarizer.
None of the prior art known to applicant discloses an efficient, compact coated prism arrangement which serves to polarize and correct the nonuniform intensity distribution of semiconductor laser beams.